Insulin-mediated effects on glycogen homeostasis in skeletal muscle: There is an energetic barrier to the direct incorporation of G1P into glycogen due to concentrations of factors that favor breakdown of glycogen under normal conditions over synthesis of glycogen. In anaerobic respiration, oxygen is not required.
The net result is an extremely large induction of glycogen breakdown in response to glucagon binding to its receptors on hepatocytes. A unicellular organism called an amoeba. First, the ligand can leave the receptor. Second, the G proteins have an inherent GTPase activity that serves to turn them off over time. For the next 8—12 hours, glucose derived from liver glycogen is the primary source of blood glucose used by the rest of the body for fuel.
Digestion is the breakdown of carbohydrates to yield an energy rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: In aerobic respiration, oxygen is required.
In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: There are several different types of carbohydrates: The breakdown of glucose into energy in the form of molecules of ATP is therefore one of the most important biochemical pathways found in living organisms.
Glycolysis can be either an aerobic or anaerobic process. When oxygen is present, glycolysis continues along the aerobic respiration pathway. If oxygen is not present, then ATP production is restricted to anaerobic respiration. The location where glycolysis, aerobic or anaerobic, occurs is in the cytosol of the cell. In glycolysis, a six-carbon glucose molecule is split into two three-carbon molecules called pyruvate. For the glucose molecule to oxidize into pyruvate, an input of ATP molecules is required.
This is known as the investment phase, in which a total of two ATP molecules are consumed. Even though ATP is synthesized, the two ATP molecules produced are few compared to the second and third pathways, Krebs cycle and oxidative phosphorylation. Even if there is no oxygen present, glycolysis can continue to generate ATP. In alcohol fermentation, when a glucose molecule is oxidized, ethanol ethyl alcohol and carbon dioxide are byproducts.
Each pyruvate releases a carbon dioxide molecule, turning into acetaldehyde. In lactic acid fermentation , each pyruvate molecule is directly reduced by NADH. The only byproduct from this type of fermentation is lactate.
Lactic acid fermentation is used by human muscle cells as a means of generating ATP during strenuous exercise where oxygen consumption is higher than the supplied oxygen. As this process progresses, the surplus of lactate is brought to the liver , which converts it back to pyruvate.
However, the activity of hexokinase in muscle is so high that any free glucose is immediately phosphorylated and enters the glycolytic pathway.
Indeed, the precise reason for the temporary appearance of the free glucose from glycogen is the need of the skeletal muscle cell to generate energy from glucose oxidation, thereby, precluding any chance of the glucose entering the blood. For de novo glycogen synthesis to proceed the first few glucose residues are attached to a protein known as glycogenin. Glycogenin functions as a homodimer and catalyzes its own glycosylation, attaching C-1 of a UDP-glucose to a tyrosine residue on the enzyme.
This reaction is carried out by one subunit adding the glucose to the other subunit. The attached glucose then serves as the primer required by glycogen synthase GS to attach additional glucose molecules via the mechanism described below.
The GYG1 gene is located on chromosome 3q24 and is composed of 10 exons that generate three alternatively spliced mRNAs. These three mRNAs produce three glycogenin-1 isoforms identified as isoform 1 amino acids , isoform 2 amino acids , and isoform 3 amino acids.
The GYG1 gene is predominantly expressed in muscle but is also expressed in many other tissues as well. Mutations in the GYG1 gene are associated with the recently characterized glycogen storage disease identified as type 15 GSD The GYG2 gene is located on chromosome Xp The GYG2 gene expression in restricted to the liver.
Reactions of the addition of glucose to glycogen: Beginning with free glucose, several reactions are required to initiate and then produce glycogen polymers. Glucose is first phosphorylated by hexokinases e. The resultant UDP-glucose can then be use as a substrate for the self-glucosylating reaction of glycogenin, or if pre-exisiting glycogen polymers exist, the UDP-glucose is utilized as the substrate for glycogen synthase.
Like the action of glycogenin, glycogen synthase utilizes UDP-glucose as its substrate. Glycogen synthase add glucose residues from UDP-glucose to terminal glucose on glycogenein as well as to the non-reducing end glucose of a molecule of glycogen. The activation of glucose to be used for glycogen synthesis is carried out by the enzyme UDP-glucose pyrophosphorylase 2.
This enzyme exchanges the phosphate on C-1 of glucosephosphate for UDP. The energy of the phosphoglycosyl bond of UDP-glucose is utilized by glycogen synthase to catalyze the incorporation of glucose into glycogen.
UDP is subsequently released from the enzyme. The human UDP-glucose pyrophosphorylase 2 enzyme is encoded by the UGP2 gene that is located on chromosome 2p15 and is composed of 14 exons that generate two alternatively spliced mRNAs. These two mRNAs encode two different isoforms of the enzyme, isoform a amino acids and isoform b amino acids.
There are two distinct glycogen synthase enzymes in humans. One is more widely expressed and predominates in skeletal muscle, the other predominates in the liver.
The broadly expressed enzyme is encoded by the GYS1 gene and the liver, heart, and pancreas enzyme is encoded by the GYS2 gene. The GYS1 gene is located on chromosome 19q Isoform 1 is composed of amino acids and isoform 2 is composed of amino acids. The GYS2 gene is located on chromosome 12p This enzyme transfers a terminal fragment of glucose residues from a polymer at least 11 glucose residues long to an internal glucose residue at the C-6 hydroxyl position.
The GBE1 gene is located on chromosome 3p Functional glycogen phosphorylase is a homodimeric enzyme that exist in two distinct conformational states: Phosphorylase is capable of binding to glycogen when the enzyme is in the R state. This conformation is enhanced by binding of AMP allosteric activator and inhibited by binding of ATP or glucosephosphate allosteric inhibitors. The enzyme is also subject to covalent modification by phosphorylation as a means of regulating its activity.
The relative activity of the unmodified phosphorylase enzyme given the name phosphorylase b is sufficient to generate enough glucosephosphate, for entry into glycolysis, for the production of sufficient ATP to maintain the normal resting activity of the cell.
This is true in both liver and muscle cells. Pathways involved in the regulation of glycogen phosphorylase. See the text for details of the regulatory mechanisms. Green arrows denote positive effects on any enzyme. Red T-lines indicate inhibitory actions. Phosphorylase kinase is itself phosphorylated, leading to increased activity, by PKA itself activated through receptor-mediated mechanisms. PKA-mediated phosphorylation of PTG results in the dissociation of the catalytic PP1 activity, the consequences of which are inhibition of phosphate removal allowing the activated enzymes to remain so for a longer time frame.
Calcium ions can activate phosphorylase kinase even in the absence of the enzyme being phosphorylated. This allows, as an example, neuromuscular stimulation by acetylcholine to lead to increased glycogenolysis in the absence of G-protein coupled receptor GPCR stimulation. It is also important to note that although this Figure only shows the regulatin of glycogen phosphorylase, all of the enzymes of glycogen breakdown and glycogen synthesis discussed below are associated in a large complex allowing for their rapid regulation.
Glucagon receptors are only found on one other cell type, white adipocytes, but at significantly lower levels than those seen on heptocytes. Because of this distribution of receptors, it is easy to understand why liver cells are the primary target for the action of glucagon. The response of cells to the binding of glucagon to its cell surface receptor is, therefore, the activation of the enzyme adenylate cyclase.
Binding of cAMP to the regulatory subunits of PKA leads to the release and subsequent activation of the catalytic subunits. The catalytic subunits then phosphorylate a number of proteins on serine and threonine residues. In this example glucagon binds to its receptor in the plasma membrane of hepatocytes, thereby activating the receptor. Activation of the receptor is coupled to the activation of the receptor-coupled heterotrimeric G-protein GTP-binding and hydrolyzing protein.
The cAMP thus produced then binds to the regulatory subunits of PKA leading to dissociation of the associated catalytic subunits. The catalytic subunits are inactive until dissociated from the regulatory subunits. The phosphorylation of phosphorylase kinase activates the enzyme which in turn phosphorylates the less active b form of phosphorylase.
Phosphorylation of phosphorylase b greatly enhances its activity towards glycogen breakdown. The modified enzyme is called phosphorylase a. The net result is an extremely large induction of glycogen breakdown in response to glucagon binding to its receptors on hepatocytes. There are two isoforms of phosphorylase kinase, one expressed in skeletal muscle and the other expressed in the liver. Both isoforms of phosphorylase kinase are multi-subunit hexadecameric enzymes composed of four copies of each of the unique subunits: The PHKG1 gene is located on chromosome 7p The PHKG2 gene is located on chromosome 16p The various Glycogen Storage diseases are listed in the Table at the end of this page.
This identical cascade of events, responsible for the regulation of glycogen phospharyylase activity, occurs in skeletal muscle cells as well. However, in these cells the induction of the cascade is the result of epinephrine binding to adrenergic receptors on the surface of muscle cells or as a result of acetylcholine binding nicotinic acetylcholine receptors at a neuromuscular junction.
Epinephrine is released from the adrenal glands in response to sympathetic nervous system outflow from the brain indicating an immediate need for enhanced glucose utilization in muscle, the so called fight-or-flight response. Muscle cells lack glucagon receptors and therefore, do not respond in any way to pancreatic effects of low blood glucose.
The presence of glucagon receptors on muscle cells would be futile anyway since the role of glucagon release is to increase blood glucose concentrations and muscle glycogen stores cannot contribute to blood glucose levels.
Calmodulin is a calcium binding protein. This activity is crucial to the enhancement of glycogenolysis in muscle cells where muscle contraction is induced via acetylcholine stimulation at the neuromuscular junction. Thus, not only does the increased intracellular calcium increase the rate of muscle contraction it increases glycogenolysis which provides the muscle cell with the glucose it needs to oxidize to satisfy the increased ATP it needs for contraction.
See the text below for details of the regulatory mechanisms. DAG binds to and activates protein kinase C PKC , an enzyme that phosphorylates numerous substrates, one of which is glycogen synthase see below. The ITPR1 gene which is located on chromosome 3p ITPR1 isoform 1 is a amino acid protein, isoform 2 is a amino acid protein, and isoform 3 is a amino acid protein.
The ITPR2 gene is located on chromosome 12p11 and is composed of 60 exons that encode a amino acid protein. The ITPR3 gene is located on chromosome 6p21 and is composed of 61 exons that encode a amino acid protein.
Each of the IP3 receptors possesses a cytoplasmic N-terminal ligand-binding domain and is comprised of six membrane-spanning helices that forms the core of the ion pore. In order to terminate the activity of the enzymes of the glycogen phosphorylase activation cascade, once the needs of the body are met, the modified enzymes need to be un-modified. The removal of the phosphates on phosphorylase kinase and phosphorylase a is carried out by a family of enzy,es identified as phosphoprotein phosphatase-1 PP1.
Each functional PP1 is a heterodimeric enzyme composed of a catalytic subunit and a regulatory subunit. There are at least 29 PP1 regulatory subunit genes expressed in the human genome. Several of the regulatory subunits are also involved in targeting of PP1 to glycogen.
These regulatory subunits are also commonly referred to as protein targeting to glycogen, PTG see Figure below. The PTG regulator of the muscle isoform of PP1 is encoded by the protein phosphatase 1, regulatory subunit 3A gene located on chromosome 7q In order that the phosphate residues placed on various enzymes by PKA and phosphorylase kinase PHK are not immediately removed, the activity of PP1 must also be regulated.
Since the protein encoded by this gene inhibits the activity of PP1 it was once called phosphoprotein phosphatase inhibitor 1 PPI-1 or PP1 inhibitor. Conversely, insulin mediated signaling results in phosphorylation of the PPP1R3A protein at Ser46 which results in increased activity of PP1, removal of the inhibitory phosphate from glycogen synthase and a subsequent increase in glycogen synthesis.
Glycogen synthase is a tetrameric enzyme consisting of four identical subunits. As indicated above, the liver glycogen synthase is encoded by the GYS2 gene while the muscle form as well as that expressed in several other tissues is encoded by the GYS1 gene. Regardless of tissue of expression, the activity of glycogen synthase is regulated by both allosteric effectors and by phosphorylation of serine residues in the subunit proteins.
Allosteric regulation of glycogen synthase activity is effected by glucosephosphate and ATP with glucosephosphate being a positive effector and ATP being and inhibitory effector. Phosphorylation of glycogen synthase has been shown to occur on at least nine different serine residues and these phosphorylations are carried out by numerous different kinases.
Phosphorylation of glycogen synthase reduces its activity towards UDP-glucose. When in the phosphorylated state, glycogen synthase inhibition can be overcome by the presence of the allosteric activator, glucosephosphate. The two forms of glycogen synthase are identified by the same nomenclature as used for glycogen phosphorylase. The unphosphorylated and most active form is glycogen synthase a and the phosphorylated, less active form is glycogen synthase b.
Numerous kinases have been shown to phosphorylate and regulate both hepatic and muscle forms of glycogen synthase. Most detailed analyses of glycogen synthase phosphorylation have been carried out using enzyme isolated from skeletal muscle but related findings with liver glycogen synthase have also been demonstrated.
At least nine sites of phosphorylation have been identified in glycogen synthase and these nine sites are clustered into four phosphorylation domains present in the N-terminal and C-terminal ends of the enzyme.
Phosphorylation of glycogen synthase occurs through the activities of at least ten distinct kinases. The phosphorylation sites in glycogen synthase are identified as site 1a, 1b, 2, 2a, 3a, 3b, 3c, 4, and 5 with sites 2, 2a, 3a, and 3b being the most significant with respect to the regulation of enzyme activity.
Regulation of glycogen synthase by phosphorylation occurs via both primary and secondary phosphorylation events. Recent studies demonstrate that the association of AMPK and PHK with the liver form of glycogen synthase is not significant or does not occur.
When glucagon binds its receptor on hepatocytes the resultant rise in activity of PKA leads to increased phosphorylation of glycogen synthase directly by PKA. The regulatory phosphorylation sitesin glycogen synthase targeted by PKA are 1a, 1b, and 2. Within liver glycogen phosphorylase phosphorylation of site 2 has been shown to be the most significant relative to its regulation.
Activated PKA also leads to phosphorylaiton and activation of phosphorylase kinase which also phosphorylates glycogen synthase on site 2. In addition, glucagon signaling effects an increase in the activity of CK2. Thus, the net effect of glucagon action on hepatocytes is activation of three distinct kinases that phosphorylate and inhibit glycogen synthase.
Pathways involved in the regulation of glycogen synthase by various kinases: PPI-1 is phosphoprotein phosphatase-1 inhibitor.
Iamges: is the production of glycogen from glucose an anabolic reaction
This occurs through dehydration synthesis reactions. Insulin also exerts a negative effect on the activity of GSK3 such that there is a reduced level of phosphorylation of glycogen synthase by this kinase. Various inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown.
Glucose obtained from these two primary sources either remains soluble in the body fluids or is stored in a polymeric form, glycogen.
Estradiol is the predominant sex hormone present in females. The various Glycogen Storage diseases are listed in the Table at the end of this page. The glucose is removed from glycogen is an activated state, i. Glycogen phosphorylase is regulated by both allosteric factors and by covalent modification phosphorylation. This activity is crucial to the enhancement of glycogenolysis in muscle cells where muscle contraction is induced via acetylcholine stimulation at the neuromuscular junction. Reconversion of glycogen synthase b to glycogen synthase a requires dephosphorylation. In humansglycogen corticosteroids mechanism of action in cancer made and stored primarily in the cells of the liver and skeletal muscle.